I once had Paper as the secondary icon theme, but I wasn't really satisfied with it. Many icons didn't look like the original icons at all, which is one of the things my icon pack tries to achieve. This and some other things made the other icons look a bit out of place. So I decided to just use the default one. You can however set a secondary icon pack yourself if you really want this.

I personally use and develop the icons with a light theme but it should work fine with both light and dark themes. Some dark icons however can look a bit less noticeable on a dark background. I just created a light/dark toggle on the website (https://flipflop97.github.io/Mato/) so you can see for yourself how it looks.
- Jul 12 2017